This invention resides in improvements in methods for coextruding of multiple layer polymeric films. It also resides in the apparatus used in the novel methods, in the novel films made therewith, and in packages made from those films. As used herein "structure" means and includes multiple layer film, multiple layer sheet, and packages made with the multiple layer films and the multiple layer sheets of the invention.
Vinylidene chloride copolymers (VDC) provide barrier to transmission of moisture vapor and oxygen and therefore are desirable for use when those properties are important. The extrusion of VDC to form tubular films has previously been practiced with annular dies having crosshead type mandrels. Spiral type mandrels are not generally used with VDC because the dwell time is desirably minimized when extruding VDC, and the amount of low shear areas in the die is desirably minimized, to prevent, or delay, the degradation of the VDC. Spiral mandrels do not generally possess these characteristics.
It is commonly known that the extrusion of VDC and other thermally degradable polymers such as EVOH and Nylon is accompanied by a plurality of processing problems. These processing problems generally relate to the sensitivity of the thermally degradable polymers to the combination of elevated temperature of the polymers contact between the polymer and hot metal, and the amount of time for which the high temperature contact is maintained between the polymer and the metal surfaces of the extrusion processing equipment during the extrusion process. For purposes of convenience the abbreviation VDC will be used for vinylidene chloride copolymers below. However, one skilled in the art will understand that the benefits of the present invention can be achieved with other thermally degradable polymers including EVOH and Nylon.
One problem with crosshead mandrel-type dies is that the VDC tends to leak into the fitment area above the crosshead groove of the annular die. There it degrades, and washes back out as carbon, typically carbon particles, into the groove and subsequently into the film-forming channel, especially along the weld line of the film. The direction of flow of the leaking polymer is a generally into the fitment area between the mandrel and the die outer wall, and then back into the channel near the weld line. The problem of leakage into the fitment area can be addressed by proving close tolerances above the groove, between the inside surface of the outer containing wall of die and the outside surface of the mandrel. Alternatively, the mandrel and the outer containing wall can be cooperatively tapered to provide a tighter fit. A shrink fitting arrangement can also be used. Both the taper fit and shrink fit techniques are, however, susceptible to accelerated wear and early failure. Further, shrink fitting arrangements take longer to disassemble and reassemble, such as for cleaning.
Another problem, which is generic to VDC extrusion, is that carbon generally can and does form in all areas where the VDC contacts metal in the die. As a processing run proceeds, carbon deposits develop in the die and, in an annular die, eventually become thick enough to affect the distribution of the polymer around the circumference of the die; as well as affecting the flow rate and the associated back pressure at the extruder. The build-up of carbon is usually experienced in a mandrel-type die as a progressive function related to the time during which the die is continuously in use. The die generally must be shut down, disassembled, and cleaned after a running period of only a few days, typically 7-10 days. In some cases the die can be purged, whereby the shutdown can be postponed for a few days. However, even purging can only postpone shutdown for a short time, measured in days, not a plurality of weeks.
The problems discussed above have generally been associated with the contact between the VDC copolymer and the die metal when both the die and the VDC copolymer are at elevated extrusion processing temperature. The above problems have been somewhat attenuated in conventional practice, for cast extruded film and sheet formed from that film, by completely encapsulating the VDC copolymer with another material (typically ethylene vinyl acetate or ethylene methyl acrylate) at the die. In conventional practice, after the VDC copolymer has been encapsulated in the encapsulating polymer (for example EVA), the encapsulated combination is then fed through a conventional cast extrusion die, such as through a coat hanger die, which forms the extruded stream, through a long slender slot, into a flat sheet of film, which is typically cast onto a metal roll for cooling.
An hypothetical problem in this type of film fabrication is that the edges of the film, being totally encapsulated, do not contain any VDC, as taught in U.S. Pat. No. 4,804,510. If these edge portions of the film are anticipated for being used, the lack of VDC in the edge portions can be a serious problem. However, since edge portions of cast extruded film are typically removed by conventional edge trimming, these areas of the film are typically not used and thus the hypothetical problem is usually eliminated by a conventional step in the cast extrusion processing.
With total encapsulation of the VDC before entering an annular die, the encapsulated layer could not form a part of the weld line as defined herein, since, by definition, its edges must touch in order to form a part of the weld line. But the encapsulated edges could not touch because of the interference of the encapsulating polymer. This would result in an area of the film, extending the full length of the film, along the crosshead weld line of the film, which would be devoid of the VDC copolymer. Since VDC is generally used because of its excellent barrier properties, either barrier to oxygen permeation, or barrier to water vapor permeation, such a gap in the protection, particularly for a tubularly shaped package, is unacceptable. While such a process could be used with trimming away of the area which is devoid of the VDC copolymer and rejoining of the trimmed edges by means of, for example, a lap seam or joint, such a process would be expensive, would necessarily open the tube, and would be economically disadvantageous as compared to other extrusion processes desired for forming a tube. Further such a process would sacrifice a significant advantage of circumferential unity normally obtained from tubular extrusion processes. Total encapsulation would also preclude conventional die oscillation for the purpose of distributing thickness variations. Yet the encapsulation concept provides a potentially significant advantage in that the frequency of the shut downs for cleaning of the die is reduced.
The sensitivity of VDC to the extrusion process, especially the tubular extrusion process, has resulted in the development of only limited-size commercial dies for fabrication of tubular films, with typical die sizes being less than 25 cm. (10 inches) circumference at the die opening. The larger the circumference of the die, the greater the amount of time that the polymer spends at the elevated temperature in the die. In traversing the die, the polymer travels around the circumference of the crosshead mandrel and/or the die channel leading to the die exit orifice, all in the process of being fabricated into a film. Thus processes for fabrication of films greater than 25 cm. (10 inches) in circumference in a transverse direction, perpendicular to the machine direction, at the die exit, and containing VDC, are generally carried out with a slot die of the cast extrusion type, followed by fabrication of the tube in a converting process. While known art indicates that circumferences at the die orifice, up to 79.8 cm. have been developed, as in 4,379,117, Baird Jr. et al, such dies at present have limited applicability and are not in wide-spread use.
To the extent tubular film processing conditions are important to achieving the desired film properties, and especially at low blow-up ratios, the tube size has heretofore been undesirably limited by the sensitivity of the VDC copolymer, or some other material has had to be substituted for the VDC copolymer. Namely, large size tubes of tubularly extruded films containing VDC could not be fabricated on a commercial scale, if at all. Indeed, commercial scale tubular fabrication has been limited to small diameter tubes such as those fabricated with dies about 10 cm. (4 inches) in diameter or less.
Thus it is desirable to provide methods for fabricating tubular multiple layer films containing a layer of VDC, which methods overcome the above problems.
It is further desirable to overcome especially the problem of the VDC copolymer getting into the fit area above the crosshead groove.
It is still further desirable to reduce the formation of carbon in the die, from degradation of VDC.
Still another desire is to reduce the fraction of the internal surface area of the die which is exposed to the VDC copolymer.
Provisions for the above improvements are taught in the above-identified applications, Ser. Nos. 07/140, 096 (now U.S. Pat. No. 4,944,972) and 07/204,485 abandoned.
It is an object of this invention to provide a method which accommodates early joinder of melt streams while providing assured layer configuration and improved uniformity of layer thicknesses, about the die circumference, at the die exit.
It is another object of the invention to provide apparatus compatible with the novel methods.
It is still another object to provide novel structures made by the methods and apparatus of the invention.